In known local area communication systems, each of a plurality of user devices is connected to the communication system buses via a bus interface unit (BIU). Each BIU, using a particular media access protocol, occasionally places an information packet from its associated user device on a transmission bus of the system, for transmission to other user devices via their respective BIUs. The instant at which a BIU places the information packet from its user device on the transmission bus is determined by the particular media access protocol that is being used. Bus accessing protocols generally fall into either of two categories, i.e., token passing or contention.
Generally, explicit token passing methods employ a token signal which is passed from BIU to BIU in a given order within the communication system, each BIU being permitted to transmit information packets only after receiving the token signal. Thereafter, each BIU transmits the token signal such that another BIU is permitted to transmit its information packets. Token passing methods substantially preclude collisions between information packets. As a penalty, however, they incur unacceptable delays during periods where few BIUs have information packets to transmit and, therefore, those few BIUs having packets to transmit must wait until the token signal is passed among the plurality of BIUs not having packets to transmit before gaining access to the transmission bus.
Contention methods are those in which each BIU may gain access to the transmission bus for transmitting its information packet at any time the subject BIU determines the transmission bus is idle. When more than one BIU attempts to gain access to the transmission bus simultaneously, a collision occurs and each BIU, generally, discontinues its transmission for a randomly selected interval after which it reattempts to gain access. Systems practicing contention methods, however, incur unacceptable delays at times when many BIUs have information packets to transmit since at such times many collisions occur. Additionally, such systems require including in the BIU means for sensing when the transmission bus is idle as well as means for generating the random time interval following a collision.
Where a communication system carries voice encoded signals, it is desirable to minimize delays experienced by BIUs in gaining access to the transmission bus. In such cases, it is especially desirable to minimize the variance in the delays experienced in user to user communications, regardless of user location on the communication system.
As described above, both token passing and contention methods experience transmission delays that increase in magnitude with the number of users, and hence BIUs, utilizing the communication system. However, the significant installed cost of transmission media, i.e. coaxial or fiber optic cables, dictates that maximum utilization be made of the medium. Thus, the need exists for a communication system in which user utilization can be maximized without suffering unacceptable transmission delays.